1. Field of the Invention
The present invention relates to a self-excited inverter including a pair of switching devices which are turned alternately on and off for generating an AC voltage as well as to a liquid crystal display (hereinafter referred to as LCD) television apparatus employing the self-excited inverter.
2. Description of the Related Art
An LCD television apparatus often employs an inverter for converting DC power into AC power as a power source for driving a cold cathode fluorescent lamp (CCFL) which is used as a light source for backlighting an LCD panel. Japanese Registered Utility Model Nos. 3112144 and 3119099 and Japanese Laid-open Patent Application Nos. 2001-110582, 2002-334779 and 2005-332710 describe inverters for driving illumination devices. The inverters described in Japanese Registered Utility Model Nos. 3112144 and 3119099 and Japanese Laid-open Patent Application No. 2002-334779 are self-excited inverters, while the inverters described in Japanese Laid-open Patent Application Nos. 2001-110582 and 2005-332710 are separately-excited inverters.
FIG. 3 is a circuit diagram showing an example of a self-excited inverter conventionally used as a power source for driving a backlight. The self-excited inverter 8a includes a pair of switching devices Q1, Q2, which may be field effect transistors (hereinafter referred to as FETs), for example, and a transformer T whose primary side is connected to the individual switching devices Q1, Q2 and secondary side is connected to a lamp 6, such as a CCFL. The transformer T has a pair of main windings W1, W2 and a drive winding W3 wound on the primary side. The switching devices Q1, Q2 are connected to the two main windings W1, W2, respectively. In FIG. 3, W4 is a secondary winding of the transformer T, R1 and R3-R8 are resistors, C is a capacitor, L is a resonating coil, and ZD is a Zener diode.
In this self-excited inverter 8a, the switching devices Q1, Q2 are alternately turned on and off by applying a pulse voltage alternately to gates of the switching devices Q1, Q2 through the drive winding W3. As a result, AC voltage is supplied to the secondary side of the transformer T through the main windings W1 and W2 for lighting the lamp 6.
When starting up the self-excited inverter 8a of FIG. 3, an input voltage (DC voltage) Vin is applied to the gates of the switching devices Q1, Q2 as a startup voltage through the resistors R1, R3-R6. This startup voltage is clamped at a constant voltage level by the Zener diode ZD. Specifically, a point y shown in FIG. 3, which is a connecting point between the resistor R1 and the Zener diode ZD, is constantly held at a fixed potential. This arrangement is adopted in the self-excited inverter disclosed in the aforementioned Japanese Registered Utility Model No. 3112144 as well; a startup voltage applied to gates of switching devices is clamped by a Zener diode (refer to FIG. 3 of the pertinent Utility Model application).
If the self-excited inverter 8a is configured to clamp the startup voltage at the constant voltage level as mentioned above, however, the startup voltage applied to the gates of the switching devices Q1, Q2 would not vary even when the input voltage Vin is reduced to regulate illumination level by the lamp 6 for reducing LCD screen brightness. On the other hand, a pulse voltage produced by the drive winding W3 drops when the input voltage Vin is reduced. Consequently, the startup voltage applied to the switching devices Q1, Q2 may become higher than the voltage produced by the drive winding W3, potentially causing the switching devices Q1, Q2 to turn on simultaneously. If the two switching devices Q1, Q2 turn on simultaneously in this way, it becomes impossible for the self-excited inverter 8a to maintain a state of normal self-excited oscillation, possibly leading to burnout of the switching devices Q1, Q2 in extreme cases. This will result in interruption of oscillation of the self-excited inverter 8a and extinction of the lamp 6.